The present invention relates generally to carburetors for internal combustion engines, and more specifically to carburetor primer mechanisms for use in internally vented float bowl carburetors.
In small internal combustion engines, and in particular those engines which are started by hand cranking, it is frequently desirable to prime the engine by introducing a fuel-rich mixture into the engine intake system to aid in starting the engine. Such priming arrangements are particularly desirable for use in internal combustion engines of the type which are commonly used in lawnmowers, snowblowers and the like, that are likely to be started either infrequently or in cold weather.
Known priming arrangements are frequently in the form of an operator actuable bulb which, when depressed, displaces a volume of air into the airspace above a carburetor float bowl or fuel well. This air exerts pressure on the fuel which forces the fuel upwardly through a conduit into the venturi, where it is mixed with air and then drawn into the intake manifold of the engine. Such priming arrangements are well known in the art and include, for example, U.S. Pat. Nos. 4,197,825; 4,203,405; 4,404,933 and 4,679,534; all assigned to the assignee of the present invention. The first two patents listed above illustrate priming arrangements wherein fuel is displaced from a fuel well, whereas the latter two patents illustrate priming arrangements wherein the air displaces fuel from the float bowl.
In general, well type priming arrangements are considered inferior to float bowl type arrangements. A problem associated with well priming systems is that, after a priming operation, fuel is only slowly replaced in the well. Therefore, if the operator attempts to prime the engine in rapid succession, no fuel is present in the well after the first priming operation so that the only effective priming operation is the first such operation. Furthermore, each prime is limited to the volume of the well, which is quite small compared to the bowl. In a float bowl type of primer, such as that found in the present invention, the volume of the priming charge may be as large as the volume of air displaced from the primer bulb. Since the primer bulb is either wholly or partially external to the carburetor body, it may therefore have a relatively large volume, which thereby permits concomitantly larger primes with each depression.
In small internal combustion engines having float bowl priming arrangements, the primer is generally activated when the operator depresses a primer bulb which displaces a volume of air into the airspace above the float bowl. Pressure exerted by this air upon the fuel in the bowl causes fuel to be forced upwardly through a nozzle into the fuel/air mixture passage, or venturi, from which this rich fuel/air mixture is drawn into the intake manifold to aid in starting the engine. After the engine has been started, a continuous flow of fuel from the bowl to the venturi must be provided in order to assure smooth operation of the engine. Therefore, it is necessary to provide a means for venting the airspace above the float bowl. This venting may be either internal whereby the air supply to the vent is drawn from the throat of the carburetor, or external whereby the venting air is supplied from the atmosphere external to the carburetor. In the past, certain problems have arisen with regard to the venting of the float bowl which have hindered the efficient operation of the engine. For example, a disadvantage associated with both internally and externally vented carburetor float bowls is that a portion of the air forced into the float bowl as a result of the priming charge escapes through the venting passage, and thus is not available to perform useful work in displacing fuel from the float bowl into the fuel/air mixture passage. Thus, the effectiveness of the priming charge has been diluted.
Additional disadvantages have been associated with the use of externally vented float bowls. For example, when the airspace above the float bowl is externally vented to the outside atmosphere there is a likelihood that contaminant dirt and water particles will be introduced into the carburetor through this vent. These contaminants may eventually clog the fuel and air passageways, and thus prevent the smooth flow of fuel to the venturi. Externally vented float bowls have been reported in the prior art that include air filters and cleaners in an attempt to solve the problems associated with contaminants entering the carburetor. Although these filters and cleaners may be successful in preventing the entry of contaminants into the carburetor, they must be frequently cleaned and/or replaced which requires an undesirable amount of time to be spent on maintenance of the engine. A further disadvantage to externally vented carburetors is that when the air cleaner becomes dirty and clogged, the difference between the air pressure in the carburetor throat and the ambient air pressure will cause the engine to run rich. Accordingly, it is preferred to employ an internally vented float bowl which avoids the above-described problems.
Internally vented float bowls have also been known in the art. Internally vented carburetor bowls are advantageous with respect to externally vented bowls in that the vent passageway leads from the space above the fuel in the bowl to the throat of the carburetor. Thus, when the air cleaner becomes clogged and the pressure within the carburetor throat decreases, the pressure above the fuel in the float bowl also decreases due to the passageway connecting the carburetor throat and bowl. This prevents rich operation of the carburetor. Internally vented bowls have a further advantage over externally vented float bowls because the air that is supplied to the vent comes from the interior of the engine, and thus has already passed through the carburetor air filter. Thus, the likelihood of introducing additional contaminants into the carburetor of an internally vented float bowl carburetor is greatly reduced.
Prior art internally vented bowls, however, generally have a venting passageway that leads directly from the bowl to the throat area of the carburetor. Thus, as previously discussed, a portion of the priming charge will escape from the airspace above the float bowl through this internal vent, thereby diluting the effectiveness of the primer. In order to attempt to eliminate this loss of priming charge, the type of internal vent that is generally found in the art is necessarily of small diameter in order to hinder this loss of the priming charge. It is difficult to calibrate this type of carburetor due to bowl vacuum created because of the small diameter vent.
It is desired to provide a vent for a carburetor that is effective in venting a float bowl or fuel well, but does not introduce contaminants into the carburetor or require frequent maintenance of the air filter. Further, it is desired to provide a vent for a carburetor that provides effective venting of the float bowl or fuel well to near atmospheric pressure, yet does not allow the priming charge to escape through the venting aperture or cause difficulties in the calibration of the carburetor.